The present invention relates generally to medical devices and is particularly directed to a fluid dispensing device and a fluid sampling device of the type which, in one embodiment penetrates the stratum corneum and epidermis, but not into the dermis of skin, and in another embodiment penetrates into the dermis, so as to interface with blood or other biological fluids. The invention is specifically disclosed as an array of microneedles which painlessly and with minimal trauma to the skin enable fluid transfer either into a body as a dispensing device, or from the body to sample body fluid.
Topical delivery of drugs is a very useful method for achieving systemic or localized pharmacological effects. The main challenge in transcutaneous drug delivery is providing sufficient drug penetration across the skin. The skin consists of multiple layers starting with a stratum corneum layer about (for humans) twenty (20) microns in thickness (comprising dead cells), a viable epidermal tissue layer about seventy (70) microns in thickness, and a dermal tissue layer about two (2) mm in thickness.
The thin layer of stratum corneum represents a major barrier for chemical penetration through skin. The stratum corneum is responsible for 50% to 90% of the skin barrier property, depending upon the drug material""s water solubility and molecular weight. The epidermis comprises living tissue with a high concentration of water. This layer presents a lesser barrier for drug penetration. The dermis contains a rich capillary network close to the dermal/epidermal junction, and once a drug reaches the dermal depth it diffuses rapidly to deep tissue layers (such as hair follicles, muscles, and internal organs), or systemically via blood circulation.
Current topical drug delivery methods are based upon the use of penetration enhancing methods, which often cause skin irritation, and the use of occlusive patches that hydrate the stratum corneum to reduce its barrier properties. Only small fractions of topically applied drug penetrates through skin, with very poor efficiency.
Convention methods of biological fluid sampling and non-oral drug delivery are normally invasive. That is, the skin is lanced in order to extract blood and measure various components when performing fluid sampling, or a drug delivery procedure is normally performed by injection, which causes pain and requires special medical training. An alternative to drug delivery by injection has been proposed by Henry, McAllister, Allen, and Prausnitz, of Georgia Institute of Technology (in a paper titled xe2x80x9cMicromachined Needles for the Transdermal Delivery of Drugs), in which an array of solid microneedles is used to penetrate through the stratum corneum and into the viable epidermal layer, but not to the dermal layer. In this Georgia Tech design, however, the fluid is prone to leakage around the array of microneedles, since the fluid is on the exterior surface of the structure holding the microneedles.
Another alternative to drug delivery by injection is disclosed in U.S. Pat. No. 3,964,482 (by Gerstel), in which an array of either solid or hollow microneedles is used to penetrate through the stratum corneum, into the epidermal layer, but not to the dermal layer. Fluid is to be dispensed either through hollow microneedles, through permeable solid projections, or around non-permeable solid projections that are surrounded by a permeable material or an aperture. A membrane material is used to control the rate of drug release, and the drug transfer mechanism is absorption. The microneedle size is disclosed as having a diameter of 15 gauge through 40 gauge (using standard medical gauge needle dimensions), and a length in the range of 5-100 microns. The permeable material may be filled with a liquid, hydrogel, sol, gel, of the like for transporting a drug through the projections and through the stratum corneum.
Another structure is disclosed in WO 98/00193 (by Altea Technologies, Inc.) in the form of a drug delivery system, or analyte monitoring system, that uses pyramidal-shaped projections that have channels along their outer surfaces. These projections have a length in the range of 30-50 microns, and provide a trans-dermal or trans-mucous delivery system, which can be enhanced with ultrasound.
Another structure, disclosed in WO 97/48440, WO 97/48441, and WO 97/48442 (by ALZA Corp.) is in the form of a device for enhancing transdermal agent delivery or sampling. It employs a plurality of solid metallic microblades and anchor elements, etched from a metal sheet, with a length of 25-400 mm. WO 96/37256 (by Silicon Microdevices, Inc.) disclosed another silicon microblade structure with blade lengths of 10-20 mm. For enhancing transdermal delivery.
Most of the other conventional drug delivery systems involve an invasive needle or plurality of needles. An example of this is U.S. Pat. No. 5,848,991 (by Gross) which uses a hollow needle to penetrate through the epidermis and into the dermis of the subject""s skin when the housing containing an expansible/contractible chamber holding a reservoir of fluidic drug is attached to the skin. Another example of this is U.S. Pat. No. 5,250,023 (by Lee) which administers fluidic drugs using a plurality of solid needles that penetrate into the dermis. The Lee drug delivery system ionizes the drug to help transfer the drug into the skin by an electric charge. The needles are disclosed as being within the range of 200 microns through 2,000 microns.
Another example of a needle that penetrates into the dermis is provided in U.S. Pat. No. 5,591,139, WO 99/00155, and U.S. Pat. No. 5,855,801 (by Lin) in which the needle is processed using integrated circuit fabrication techniques. The needles are disclosed as having a length in the range of 1,000 microns through 6,000 microns.
The use of microneedles has great advantages in that intracutaneous drug delivery can be accomplished without pain and without bleeding. As used herein, the term xe2x80x9cmicroneedlesxe2x80x9d refers to a plurality of elongated structures that are sufficiently long to penetrate through the stratum corneum skin layer and into the epidermal layer, yet are also sufficiently short to not penetrate to the dermal layer. Of course, if the dead cells have been completely or mostly removed from a portion of skin, then a very minute length of microneedle could be used to reach the viable epidermal tissue.
Since microneedle technology shows much promise for drug delivery, it would be a further advantage if a microneedle apparatus could be provided to sample fluids within skin tissue. Furthermore, it would be a further advantage to provide a microneedle array in which the individual microneedles were of a hollow structure so as to allow fluids to pass from an internal chamber through the hollow microneedles and into the skin, and were of sufficient length to ensure that they will reach into the epidermis, entirely through the stratum corneum.
Accordingly, it is a primary advantage of the present invention to provide a microneedle array in the form of a patch which can perform intracutaneous drug delivery. It is another advantage of the present invention to provide a microneedle array in the form of a patch that can perform biological body-fluid testing and/or sampling (including interstitial fluids and/or blood). It is a further advantage of the present invention to provide a microneedle array as part of a closed-loop system to control drug delivery, based on feedback information that analyzes body fluids, which can achieve real time continuous dosing and monitoring of body activity. It is yet another advantage of the present invention to provide an electrophoretically/microneedle-enhanced transdermal drug delivery system in order to achieve high-rate drug delivery and to achieve sampling of body fluids. It is a yet further advantage of the present invention to provide a method for manufacturing an array of microneedles using microfabrication techniques, including standard semiconductor fabrication techniques. It is still another advantage of the present invention to provide a method of manufacturing an array of microneedles comprising a plastic material by a xe2x80x9cself-moldingxe2x80x9d method, a micromolding method, a microembossing method, or a microinjection method. It is still another advantage of the present invention to provide an array of edged microneedles that, in one configuration are hollow and have at least one blade with a substantially sharp edge that assists in penetration of the stratum corneum of skin, and in another configuration the microneedles are solid and have at least one blade with a substantially sharp edge to assist in penetrating the stratum corneum. It is still a further advantage of the present invention to provide a microneedle array that has sufficient separation distance between the individual microneedles so as to ensure penetration of the stratum corneum of skin to achieve greater transdermal flux. It is still another advantage of the present invention to provide a method of manufacturing an array of microneedles in which a metal mold is initially manufactured for use in a microembossing procedure, while allowing a sufficient separation distance between individual microneedles of the array, then use a procedure for creating hollow chambers and through-holes in the substrate of the microneedle array. It is yet another advantage of the present invention to provide a microneedle array that has sensing capabilities using optical, spectroscopic, colorimetric, electrochemical, thermal, gravimetric, and light scattering sensing means. It is still another advantage of the present invention to provide a method for manufacturing an array of microneedles that uses shear forces during a de-molding procedure to create sharp hollow microneedles.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a first embodiment of an improved microneedle array is constructed of silicon and silicon dioxide compounds using MEMS (i.e., Micro-Electro-Mechanical-Systems) technology and standard microfabrication techniques. The microneedle array may be fabricated from a silicon die which can be etched in a microfabrication process to create hollow or solid individual microneedles. The resulting array of microneedles can penetrate with a small pressure through the stratum corneum of skin (including skin of animals, reptiles, or other creatures-typically skin of a living organism) to either deliver drugs or to facilitate biological fluid sampling (e.g., sampling interstitial fluids and/or blood) through the hollow microneedles or pores made through skin via solid microneedles. The drug reservoir, and/or the chemical analysis components for sampling body fluid, may be fabricated inside the silicon die, or an additional thick film layer can be bonded or otherwise attached over the silicon substrate to create the reservoir. The delivery of drugs and sampling of fluids can be performed by way of passive diffusion (e.g., time release), instantaneous injection, pressure, vacuum, ultrasound, or electrophoresis (e.g., iontophoresis). A complete closed-loop system can be manufactured including active elements, such as micro-machined pumps, heaters, and mixers, as well as passive elements such as sensors. A xe2x80x9csmart patchxe2x80x9d can thereby be fabricated that samples body fluids, performs chemistry to decide on the appropriate drug dosage, and then administers the corresponding amount of drug. Such a system can be made disposable, including one with an on-board power supply.
In a second embodiment, an array of hollow (or solid) microneedles can be constructed of plastic or some other type of molded or cast material. When using plastic, a micro-machining technique is used to fabricate the molds for a plastic microforming process. The molds are detachable and can be re-used. Since this procedure requires only a one-time investment in the mold micro-machining, the resulting plastic microstructure should be much less expensive than the use of microfabrication techniques to construct microneedle arrays, as well as being able to manufacture plastic microneedle arrays much more quickly and accurately. It will be understood that such hollow microneedles may also be referred to herein as xe2x80x9chollow elements,xe2x80x9d or xe2x80x9chollow projections,xe2x80x9d including in the claims. It will also be understood that such solid microneedles may also be referred to herein as xe2x80x9csolid elements,xe2x80x9d or xe2x80x9csolid projectionsxe2x80x9d (or merely xe2x80x9cprojectionsxe2x80x9d), including in the claims.
Molds used in the second embodiment of the present invention can contain a micropillar array and microhole array (or both), which are fabricated by micro-machining methods. Such micro-machining methods may include micro electrode-discharge machining to make the molds from a variety of metals, including stainless steel, aluminum, copper, iron, tungsten, and their alloys. The molds alternatively can be fabricated by microfabrication techniques, including deep reactive etching to make silicon, silicon dioxide, and silicon carbide molds. Also, LIGA or deep UV processes can be used to make molds and/or electroplated metal molds.
The manufacturing procedures for creating plastic (or other moldable material) arrays of microneedles include: xe2x80x9cself-molding,xe2x80x9d micromolding, microembossing, and microinjection techniques. In the xe2x80x9cself-moldingxe2x80x9d method, a plastic film (such as a polymer) is placed on a micropillar array, the plastic is then heated, and plastic deformation due to gravitational force causes the plastic film to deform and create the microneedle structure. Using this procedure, only a single mold-half is required. When using the micromolding technique, a similar micropillar array is used along with a second mold-half, which is then closed over the plastic film to form the microneedle structure. The micro-embossing method uses a single mold-half that contains an array of micropillars and conical cut-outs (microholes) which is pressed against a flat surface (which essentially acts as the second mold-half) upon which the plastic film is initially placed. In the microinjection method, a melted plastic substance is injected between two micro-machined molds that contain microhole and micropillar arrays.
Of course, instead of molding a plastic material, the microneedle arrays of the present invention could also be constructed of a metallic material by a die casting method using some of the same structures as are used in the molding techniques discussed above. Since metal is somewhat more expensive and more difficult to work with, it is probably not the preferred material except for some very stringent requirements involving unusual chemicals or unusual application or placement circumstances. The use of chemical enhancers, ultrasound, or electric fields may also be used to increase transdermal flow rate when used with the microneedle arrays of the present invention.
In the dispensing of a liquid drug, the present invention can be effectively combined with the application of an electric field between an anode and cathode attached to the skin which causes a low-level electric current. The present invention combines the microneedle array with electrophoretic (e.g., iontophoresis) or electroosmotic enhancement, which provides the necessary means for molecules to travel through the thicker dermis into or from the body, thereby increasing the permeability of both the stratum corneum and deeper layers of skin. While the transport improvement through the stratum corneum is mostly due to microneedle piercing, electrophoresis (e.g., iontophoresis) provides higher transport rates in epidermis and dermis.
The present invention can thereby be used with medical devices to dispense drugs by electrophoretic/microneedle enhancement, to sample body fluids (while providing an electrophoretically/microneedle-enhanced body-fluid sensor), and a drug delivery system with fluid sampling feedback using a combination of the other two devices. For example, the body-fluid sensor can be used for a continuous or periodic sampling noninvasive measurement of blood glucose level by extracting glucose through the skin by reverse iontophoresis, and measuring its concentration using a bioelectrochemical sensor. The drug delivery portion of this invention uses the microneedle array to provide electrodes that apply an electric potential between the electrodes. One of the electrodes is also filled with an ionized drug, and the charged drug molecules move into the body due to the applied electric potential.
In an alternative embodiment of hollow microneedles, an edged microneedle is provided that includes at least one longitudinal blade that runs to the top surface or tip of the microneedle to aid in penetration of the stratum corneum of skin. The blade at the top surface provides a sharp tip that increases the likelihood of penetrating the skin when coming into contact therewith. In a preferred mode of the edged hollow microneedles, there are two such longitudinal blades that are constructed on opposite surfaces at approximately a 180xc2x0 angle along the cylindrical side wall of the microneedle. Each edged blade has a cross-section that, when viewed from above the microneedle top, has a profile that is approximately that of an isosceles triangle. The blade""s edge can run the entire length of the microneedle from its very top surface to its bottom surface where it is mounted onto the substrate, or the edge can be discontinued partway down the length of the microneedle as the microneedle outer surface approaches the substrate. The orientation of the blades in the microneedle array can be random, in which the blades of various individual microneedles point in all different directions.
In an alternative embodiment of a solid microneedle, a star-shaped solid microneedle is provided having at least one blade with a relatively sharp edge to assist in penetrating the stratum corneum of skin. In a preferred embodiment of a bladed or edged solid microneedle, a three pointed star-shaped solid microneedle is provided in which each blade has a triangular cross-section when viewed from the top of the microneedle, and each of these triangles approximates that of an isosceles triangle. The base of each of the isosceles triangles meets at a center of the microneedle to form a star-shaped structure when seen from the top of the microneedle. At least one hole through the substrate preferably is located near the side surfaces of at least one pair of blades of the solid microneedle, and preferably a through-hole would be located near each pair of such blades. In this preferred embodiment, there would be three edged blades and three adjacent through-holes in the substrate for each microneedle.
In a further alternative embodiment, a porous polymer, such as a hydrogel or solgel matrix can be impregnated with active material and deposited in the inside corners between the blades of the star. This provides an additional delivery mechanism.
The microneedle arrays of the present invention are significantly improved by using a proper separation distance between each of the individual microneedles. A very useful range of separation distances between microneedles is in the range of 100-300 microns, and more preferably in the range of 100-200 microns. The outer diameter and microneedle length is also very important, and in combination with the separation distance will be crucial as to whether or not the microneedles will actually penetrate the stratum corneum of skin. For hollow circular microneedles, a useful outer diameter range is from 20-100 microns, and more preferably in the range of 20-50 microns. For circular microneedles that do not have sharp edges, a useful length for use with interstitial fluids is in the range of 50-200 microns, and more preferably in the range of 100-150 microns; for use with other biological fluids, a useful length is in the range of 200 microns-3 mm, and more preferably in the range of 200-400 microns.
For circular hollow microneedles having sharp edges (such as those having the blades with triangular shaped edges), a useful length for use with interstitial fluids is in the range of 50-200 microns, and more preferably in the range of 80-150 microns; for use with other biological fluids, a useful length is again in the range of 200 microns-3 mm, and more preferably in the range of 200-400 microns. An example of a xe2x80x9csharp edgexe2x80x9d as used herein is where the tip of the blade edge exhibits a dimension at its angular vertex that is as narrow or narrower than 0.5 microns. For solid microneedles having a star-shaped profile with sharp edges for its star-shaped blades, a useful length is in the range of 50-200 microns, and more preferably in the range of 80-150 microns, while the radius of each of its blades is in the range of 10-50 microns, and more preferably in the range of 10-15 microns.
The present invention can be manufactured with an alternative methodology using a mold preparation procedure that begins by placing an optical mask over a layer of PMMA material, then exposing the PMMA material that is not masked to x-rays or another type of high energy radiation (e.g., neutrons, electrons), and developing that PMMA material in a photoresist process. The remaining PMMA material is then coated (e.g., electroplated) with metal, such as nickel. When the coating has reached the appropriate thickness, it is detached to become a metal mold to create polymer or other type of moldable plastic material. This metal mold is then used in a microembossing procedure, in which the metal mold is pressed against a heated layer of polymer or other plastic material. Once the mold is pressed down to its proper distance, the plastic or polymer material is cooled to be solidified, and the mold is then detached, thereby leaving behind an array of microneedles. If the microneedles are hollow, then alternative procedures to create through-holes all the way through the microneedles and its underlying substrate material uses a methodology such as, for example, laser ablation, water jet erosion, electric discharge machining, plasma etching, and particle bombardment.
Another alternative procedure to create polymer or plastic microneedles is to begin with a two-layer laminate structure of biocompatible material. A metallic mold created by any process is then pressed down all the way through the top layer of this laminate, and partially into the bottom layer to ensure that the top layer is entirely penetrated. This occurs while the laminate material has been heated to its plastic, deformable temperature. Once the laminate material has then been cooled, the mold is removed and the top layer is detached from the bottom layer. This top layer will now have holes that will be further operated upon by a microembossing procedure using a different mold. This different mold creates hollow microneedles, in which the through-holes that normally need to be later created in the substrate have already been created in advance by the first pressing or molding procedure.
Another refinement of the present invention is to create a microneedle array that has sensing capabilities. In this structure, the tips or side grooves of the microneedles are coated with a particular chemical that aids in detecting a particular chemical or biological structure or fluid that come into contact with the tips of the microneedles. A sensing means is performed by the use of optical energy, for example such as a laser light source that is directed through the microneedle structure, in which the microneedles themselves are made of substantially transparent material. Other sensing mechanisms also could be used, as discussed hereinbelow.
A further alternative manufacturing process for hollow or solid microneedles is to create shear forces along the outer surfaces of the distal or tip portion of the hollow or solid microneedle during its molding or embossing process. The shear forces are actually created during the de-molding step while the microneedle array material is being cooled. The amount of shear can be controlled by the cool-down temperature, and if properly done will result in microneedles having sharp edges (rather than smooth edges) along their upper surfaces at their tips.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.